Buffs stacks. Every job is now on 120s burst window and you want the most you can have.
Dancer also have healing and shields that can make the difference. MCH only got DPS.
Then, remember that each buff/nerf/gear progression also impact the rDPS jobs.
Finally, let me remind you that BLM is a pure DPS, RDM and SMN are support jobs but no one is complaining that the pure rDPS job is doing that much damage compared to SMN&RDM.
Using 24 man raid, dungeons and normal raid as a metric... People go in for fun, not for performances. You'll never get in an optimized party you could have no casters or people not playing very well. Unless you match with 7 other raiders, you'll never get a good party for your buffs. It's also true for normal raids. Dungeons are 4 man content, less people using your buffs means less rDPS.
You can't look at lower percentile, under 75 percentile contains people getting K.O. or not fully geared. 90+ mostly contains people that know their jobs, are geared and don't get KO.
TL;DR:
MCH brings only damage to the group.
DNC brings damage and shield/healing.
MCH doesn't have anything to compete with that support, it should get something to compete.
On top of that, it's competing for the ranged role. There is no problem having 2 casters or 2 melees, but having 2 ranged is very bad.
MNK has been dumbed down since 6.0. I'll also let you discover Dragon Kick rotation.